Chapter VI: Preface: To First Edition (6)

Newton knew that if a planet were merely left alone in space, it would continue to move on forever in a straight line. But Kepler had shown that the planet did not move in a straight line, but that it described an ellipse. One conclusion was obvious. There must be some force acting upon the planet which pulls it away from the straight line it would otherwise pursue. We may, for the sake of illustration, imagine this force to be applied by a rope attached to the planet so that at every moment it is dragged by some unseen hand. To find the direction this rope must have, we take the law of Kepler, which explains the rules according to which the planet varies its speed. I cannot enter into the question fully, as it would be too difficult for us to discuss now. I should have to talk a great deal more about mathematics than would be convenient just at present; but I think you can all understand the result to which Newton was led. He showed that the rope must always be directed towards the sun. In other words, suppose that there was no sun, but that in the place which it occupied there was a strong enough giant constantly pulling away at the planet, then we should find that the speed of the planet would alter just in the way it actually does. Thus we learn that some force must reside in the sun by which the planet is drawn, and this force is exerted, although there is no visible bond between the sun and the planet.

There is another fact to be learned about the sun’s attraction, and this time we obtain it by knowing the shape of the curve followed by the planet. The laws by which the planet’s speed is regulated prove that the force emanates from the sun. We shall now learn much more when we take into account that the path of the planet is an ellipse, of which the sun lies at the focus. Nothing has been said as yet regarding the magnitude of the pull which is being exerted by the sun. Is that pull to be always the same, or is it to be greater at some times than at other times? Newton showed that no ellipse other than a circle could be described, if the pull from the sun were always the same. Its magnitude must be continually changed, and the nearer the planet lies to the sun, the more vehement is the pull it receives. Newton laid down the exact law by which the force on the planet at any one place in its path could be compared with the force at any other position. Let us suppose that the planet is in a certain position, and that it then passes into a second position, which is twice as far from the sun. The pull upon the planet at the shorter distance is not only greater than the pull at the longer distance, but it is actually four times as much. Stating this result a little more generally, we assert, in the language of astronomers, that the _attraction varies inversely as the square of the distance_. If this law were departed from, then I do not say that it would be impossible for the planet to revolve around the sun in some fashion, but the motion would not be performed in an ellipse described around the sun in the focus.

You see how very instructive are the laws which Kepler discovered. From the first of them we were able to infer that the sun attracts the planets; from the second, we have learned how the magnitude of the attracting force varies.

The true importance of these great discoveries will be manifest when we compare them with what we have already learned with regard to the movements of the moon. As the moon revolves around the earth it is held by the earth’s attraction, and the moon follows a path which, though nearly a circle, is really an ellipse. This orbit is described around the earth just as the earth describes its path around the sun. That law by which a stone falls to the ground in consequence of the earth’s attraction is merely an illustration of a great general principle. Every body in the whole universe attracts every other body.

Think of two weights lying on the table. They no doubt attract each other, but the force is an extremely small one--so small, indeed, that you could not measure it by any ordinary appliance. One or both of the attracting masses must be enormously big if their mutual gravitation is to be readily appreciable. The attraction of the earth on a stone is a considerable force, because the earth is so large, even though the stone may be small. Imagine a pair of colossal solid iron cannon-balls, each 53 yards in diameter, and weighing about 417,000 tons. Suppose these two globes were placed a mile apart, the pull of one of them on the other by gravitation would be just a pound weight. Notwithstanding the size of these masses, the hand of a child could prevent any motion of one ball by the attraction of the other. If, however, they were quite free to move, and there was absolutely no friction, the balls would begin to draw together; at first they would creep so slowly that the motion would hardly be noticed. The pace would no doubt continue to improve slowly, but still not less than three or four days must elapse before they will have come together.

By the kindness of Professor Dewar, I am enabled to exhibit a contrivance with which we can illustrate the motion of a planet around the sun. Here is a long wire suspended from the roof of this theatre, and attached to its lower end is an iron ball, made hollow for the sake of lightness. When I draw the ball aside, it swings to and fro with the regularity of a great pendulum. But when I place a powerful magnet in its neighborhood (Fig. 59), you see that as soon as the ball gets near the magnet it is violently drawn to one side, and follows a curved path. This magnet may be taken to represent the sun, while the ball is like our earth, or any other planet, which would move in a straight line were it not for the attraction of the sun which draws the body aside.

THE GEOGRAPHY OF MARS.

We will now say something with respect to the geography of our fellow-planet, a subject which seems all the more interesting because Mars is so like the earth in many respects. We require a fairly good telescope for the purpose of seeing him well, but when such an instrument is directed to the planet, a beautiful picture of another world is unfolded (Fig. 60). There are many things visible on his surface, but we must always remember that even with our most powerful telescopes the planet still appears a long way off.

(_By Douglass, Lowell Observatory._)]

In the most favorable circumstances, Mars is at least one hundred times as far from us as the moon. But we know that an object on the moon must be as large as St. Paul’s Cathedral if it is to be visible in our telescopes. An object on Mars must be, therefore, at least one hundred times as broad and one hundred times as long as St. Paul’s Cathedral if it is to be discernible by astronomers on our earth. We can, therefore, only expect to see the general features of our fellow-planet. Were we looking at our earth from a similar distance, and with equally good telescopes, the continents and oceans, and the larger seas and islands, would all be large enough to be conspicuous. It is, however, doubtful whether they could ever be properly revealed through the serious impediment to vision which our atmosphere would offer.

It fortunately happens that the surface of Mars is only obscured by clouds to a very trifling extent, and we are thus able to see a panorama of our neighboring globe laid before us. Mars is not nearly so large as our earth, the diameters of the two bodies being nearly as two to one. It follows that the number of acres on the planet is only a quarter of the number of acres on the earth. Careful telescopic scrutiny shows that the chief features which we see on Mars are of a permanent character. In this respect Mars is much more like the moon than the sun. The latter presents to us merely glowing vapors, with hardly more permanence than is possessed by the clouds in our own sky. On the other hand, the entire absence of clouds from the moon enables us to see the permanent features on its surface. Most of the visible features on Mars are also invariable; though occasionally it would seem that the climate produces some changes in its appearance.

We first notice that there are differently colored parts on Mars. The darkish or bluish regions are usually spoken of as seas or oceans; though we should be going beyond our strict knowledge were we to assert that water is actually found there. Look at the horn-shaped object in the centre of the lower picture in Fig. 60. We call it the Kaiser Sea, and it is so strongly marked that even in a small telescope it can be often seen. You must not, however, always expect to notice this feature when you look at the planet through a telescope, for it turns round and round. We can make a globe representing Mars. On this are to be depicted this great sea and the other characteristic objects. But as we turn the globe around, the opposite side of the planet is brought into view, and other features are revealed like those represented in the upper figure. Mars requires 24 hours 37 minutes 22.7 seconds to complete a single rotation. It is somewhat remarkable that this only differs from the earth’s period of rotation by a little more than half an hour.

Mars contains what we call continents as well as oceans, and we also find there lakes and seas and straits. These objects are indicated in the drawings that are here represented. But the most striking features which the planet displays are the marvellous white regions, which are seen both at its North Pole and at its South Pole (Fig. 62). If we were able to soar aloft above our earth and take a bird’s-eye view of our own polar regions, we should see a white cap at the middle of the arctic circle. This appearance would be produced by the eternal ice and snow. It would increase during the long, dark winter, and be somewhat reduced by melting during the continuously bright summer. Though we cannot thus see our earth, yet we can sometimes observe one Pole of Mars and sometimes the other, and we find each of these Poles crowned with a dense white cap, which increases during the severity of its winter, and which declines again with the warmth of the ensuing summer.

Sketches of Mars have been made by many astronomers; among them we may mention Mr. Green, who made a beautiful series of pictures at Madeira in 1877. These may be supplemented by the drawings of Mr. Knobel in 1884, when the opposite Pole of the planet was turned to view. The drawings show the polar snows, and there seem to be some elevated districts in his arctic regions which retain a little patch of snow after the main body of the ice cap has shrunk within its summer limits. An interesting case of this kind is shown in Fig. 62, which has been copied from one of Mr. Green’s drawings.

It has lately been surmised that the continents on Mars are occasionally inundated by floods of water. There are also indications of clouds hanging over the Martian lands, but the inhabitants of that planet, in this respect, escape much better than we do. A certain amount of atmosphere always surrounds Mars, though it is much less copious than that we have here. As to the composition of this atmosphere we know nothing. For anything we can tell, it might be a gas so poisonous that a single inspiration would be fatal to us; or if it contained oxygen in much larger proportion than our air does, it might be fatal from the mere excitement to our circulation which an over-supply of stimulant would produce. I do not think it the least likely that our existence could be supported on Mars, even if we could get there. We also require certain conditions of climate, which would probably be totally different from those we should find on Mars.

Many remarkable observations of Mars have been lately made by Mr. Percival Lowell. It seems very doubtful how far our former division of continents and oceans on Mars can be maintained. Mr. Lowell has paid special attention to a wonderful system of lines on the planet’s surface to which the name of “canals” has been given, which often show such a degree of regularity as would almost suggest the idea that they had been laid down by intelligent guidance.

THE SATELLITES OF MARS.

When Mars appeared in his full splendor in 1877, he was for the first time honored with the notice of instruments capable of doing him justice. I do not, however, mean that in former apparitions he was not also carefully observed, but a great improvement had recently taken place in telescopes, and it was thus under specially favorable auspices that his return was welcomed in 1877. This year will be always celebrated in astronomical history for a beautiful discovery made by Professor Asaph Hall, the illustrious astronomer at Washington.

Before I can explain what this discovery was, I must have a little talk about moons, or satellites as they are often called. You know that we have one moon, which is constantly revolving round the earth, and accompanies the earth in its long voyage round the sun. But the earth is only a planet, and there are many other planets which are worlds like ours. It is natural to compare these worlds, and as we have one moon, why should not the other planets also have moons? If there are children in one house in a square, why should there not be children in the other houses? We find that some of the other planets have satellites, but they do not seem to be distributed very regularly. In fact, they are almost as capriciously allotted as the children would be in eight houses that you might take at random.

In Number One there lives an old bachelor, and in Number Two a single lady. These are Mercury and Venus, and of course there are no children in either of these houses. Number Three is inhabited by old mother Earth, and she has got a fine big son, called the Moon. Number Four is a nice little house inhabited by Mars. There are to be found a pair of little twins, and nimble creatures they are too. Number Five is a great mansion. A very big man lives here, called Jupiter, with four robust sons and daughters that everybody knows. I fancy they must go to many dancing parties, for every night they may be seen whirling round and round. For three hundred years these four moons have been known to astronomers, but in 1892 there was an addition to the family in the shape of a tiny moon which had never been seen up to that time. Number Six is also a fine big house, though not quite so big as Number Five, but larger than any of the others. It is inhabited by Saturn, and contains the biggest family of all. Up till the other day eight sons and daughters were known to live here, but they are not nearly so sturdy as Jupiter’s children; in fact, the young Saturns do not make much display, and some of them are so delicate that they are hardly ever seen. In this household, too, a new member has recently appeared. For fifty years the family was known to consist of these eight sons and daughters, but in August, 1898, when they were being photographed in a group, it was discovered that a ninth moon had been added. Number Seven is also a fine large house; but Uranus, who lives there, is such a recluse that unless you carefully keep your eye on his house, you will hardly ever catch a glimpse of him. There are four children in that house, I believe, but we hardly know them. They move in circles of their own, and apparently have seen a good deal of trouble. Only one more house is to be mentioned, and that is Number Eight, inhabited by Neptune. It contains one child, but we are hardly on visiting terms with this household, and we know next to nothing about it.

Before 1877, Mars appeared to be in the same condition as Venus or Mercury--that is, devoid of the dignity of attendants. There was, however, good reason for thinking that there might be some satellites to Mars, only that we had not seen them. You see that, as Number Three had one child, and Numbers Five, Six, Seven, and Eight had each one, or more than one, it seemed hard that poor Number Four should have none at all. It was, however, certain that if there were any satellites to Mars, they must be comparatively small things; for if Mars had even one considerable moon, it must have been discovered long ago.

On the memorable occasion in 1877, Professor Hall discovered that the ruddy planet Mars was attended, not alone by one moon, but by two. Their behavior was most extraordinary. It appeared to him at first almost as if one of these little moons was playing at hide-and-seek. Sometimes it would peep out at one side of the planet, and sometimes at the other side. I have here a picture (Fig. 63) which shows how these moons of Mars revolve. That is the globe of the planet himself in the middle, and he is turning round steadily in a period which is nearly the same as our day. But the remarkable point is that the inner of the moons of Mars runs round the planet in 7 hours 39 minutes. It would seem very strange in our sky if we had a little moon which rose in the west instead of in the east, and which galloped right across the heavens three times every day--and this is what Mars has. The outer moon takes a more leisurely journey, for he requires 30 hours 18 minutes to complete a circuit. If for no other reason than to see these wonderful moons, it would be very interesting to visit Mars.

The satellites of this planet are in contrast to our moon. In the first place, our moon takes 27 days to go round the earth, and is comparatively a long way off. The moons of Mars are much nearer to their planet, and they go round much more quickly. There is also another difference. The moons of Mars are much smaller bodies than our moon. If we represent Mars by a good-sized football, his moons, on the same scale, would be hardly so big as the smallest-sized grains of shot. Does it not speak well for the power of telescopes in these modern days that objects so small as the satellites of Mars should be seen at all? You remember, of course, that neither Mars himself nor his moons have any light of their own. They shine solely in consequence of the sunlight which falls upon them. They are merely lighted like the earth itself, or like the moon. The difficulty about observing the satellites is all the greater because they are seen in the telescope close to such a brilliant body as Mars. The glare from the bright planet is such that when we want to see faint objects like the satellites we have to hide Mars, so as to get a comparatively dark space in which to search.

Now that they know exactly what to look for, a good many astronomers have observed the satellites of Mars. A superb telescope is nevertheless required. And, in fact, you could not find a better test for the excellence of an instrument than to try if it will show these delicate objects. But do not imagine that merely having a good telescope and a clear sky is all that is requisite for making astronomical discoveries. You might just as well say that by putting a first-rate cricket-bat in any man’s hands will ensure his making a grand score. Every boy knows that the bat does not make the cricketer, and I can assure him that neither will the telescope make the astronomer. In both cases, no doubt, there is some element of luck. But of this you may be certain: that as it is the man that makes the score, and not the bat, so it is the astronomer that makes the discovery, and not his telescope.

Deimos and Phobos were the names of the two personages, according to Homer, whose duty it was to attend on the god Mars, and to yoke his steeds. A conclave of classical scholars and astronomers appropriately decided that Deimos and Phobos must be the names of the two satellites to the planet which bears the name of Mars.

HOW THE TELESCOPE AIDS IN VIEWING FAINT OBJECTS.

We have been hitherto talking about large planets, which, if not as big as our earth, are at least as big as our moon. But now we have to say a few words about a number of little planets, many of them being so very small that a million rolled together would not form a globe so big as this earth. These little objects you cannot see with your unaided eye, and even with a telescope they only look like very small stars.

I have often been asked why it is that a telescope enables us to see objects, both faint and small, which our unaided eyes fail to show. Perhaps this will be a good opportunity to say a few words on the subject. I think we can explain the utility of the telescope by examining our own eyes. The eye undergoes a remarkable transformation when its owner passes from darkness into a brilliantly lighted room (Fig. 64). Here you see two views of an eye, and you notice the great difference between them. They are not intended to be the eyes of two different people, or the two eyes of the same person; they are merely two conditions of the same eye. They are intended to illustrate two different states of the eye of a collier. The right shows his eye when he is above ground in bright daylight; the left is his eye when he has gone down the coal-pit to his useful work in the dark regions below. I remember when I went down a coal-pit I was lowered down a long shaft, and when the bottom was reached a safety lamp was handed to me. The gloom was such, that at first I found some little difficulty in guiding my steps, but the capable guide beside me said in an encouraging voice, “You will be all right, sir, in a few moments, for you will get your _pit-eyes_.” I did get my “pit-eyes,” as he promised, and was able to see my way along sufficiently to enjoy the wonderful sights that are met with in the depths below.

This shows the Eye in the Daylight.

FIG. 64.]

The change that came over my eyes is that which these two pictures illustrate: the black, round spot in the centre is an opening covered with a transparent window, by which light enters the eye; the black spot is called the pupil, and nature has provided a beautiful contrivance by which the pupil can get larger or smaller, so as to make vision agreeable. When there is a great deal of light we limit the amount that enters by contracting the pupil so as to make the opening smaller. Thus the picture with the small pupil represents the state of the collier’s eye when he is above ground in bright sunlight. When he descends into the pit, where the light is very scanty, then he wants to grasp as much of it as ever he can, and consequently his pupil enlarges so as to make a wider opening, and this is what he calls getting his “pit-eyes.”

But you need not go down a coal-mine to see the use of the iris--for so that pretty membrane is called which surrounds the pupil. Every time you pass from light into darkness the same thing can be perceived. When we turn down the lights in a room, so that we are in comparative darkness, our pupils gradually expand. As soon as the lights are turned up again, then our pupils begin to contract. Other animals have the same contrivance in their eyes. You may notice in the Zoölogical Gardens how quickly the pupil of the lion contracts when he raises his eyes to the light. The power of rapidly changing the pupil might be of service to a beast of prey. Imagine him crouching in a dense shade to wait for his dinner; then of course the pupil will be large from deficiency of light; but when he springs out suddenly on his victim, in bright light, it would surely be of advantage to him to be able at once to see clearly. Accordingly his pupil adjusts itself to the altered conditions with a rapidity that might not be necessary for creatures of less predaceous habits.

These changes of the pupil explain how the telescope aids our eyes when we want to discern any faint objects, like the little planets. Such bodies are not visible to the unaided eyes, because our pupils are not large enough to grasp sufficient light for the purpose. Even when they are opened to the utmost, we want something that shall enable them to open wider still. We must therefore borrow assistance from some device which shall have an effect equivalent to an enlargement of the pupil beyond the limits that nature has actually assigned to it. What we want is something like a funnel which shall transform a large beam of rays into a small one. I may explain what I mean by the following illustration: Suppose that it is raining heavily, and that you want to fill a bucket with water. If you merely put the bucket out in the middle of a field, it will never be filled; but bring it to where the rain-shoot from a house-top is running down, and then your bucket will be running over in a few moments. The reason, of course, is that the broad top of the house has caught a vast number of drops and brought them together in the narrow shoot, and so the bucket is filled. In the same way the telescope gathers the rays of light that fall on the object glass, and condenses them into a small beam which can enter the eye. We thus have what is nearly equivalent to an eye with a pupil as big as the object glass. Thus the effect of a grand telescope amounts to a practical increase of the pupil from the size of a threepenny-piece up to that of a dinner-plate, or even much larger still.

THE ASTEROIDS OR SMALL PLANETS.

An asteroid is like a tiny star, and in fact the two bodies are very often mistaken. If we could get close to the objects, we should see a wide difference between them. We should find the asteroid to be a dark planet like our earth, lighted only by the rays from the sun. The star, small and faint though it may seem, is itself a bright sun, at such a vast distance that it is only visible as a small point. The star is millions of times as far from us as the planet, and utterly different in every respect.

It is a curious fact that the planets should happen to resemble the stars so closely. We can find an analogous fact in quite another part of nature. In visiting a good entomological collection, you will be shown some of the wonderful leaf-like insects. These creatures have wings, exactly formed to imitate leaves of trees, with the stalks and veins completely represented. When one of these insects lies at rest, with its wings folded, among a number of leaves, it would be almost impossible to penetrate the disguise. This mimicry is no doubt an ingenious artifice to deceive the birds or other enemies that want to eat the insect. There is, however, one test which the cunning bird could apply: the leaves do not move about of their own accord, but the leaf-insects do. If therefore the bird will only have the patience to wait, he will see a pair of the seeming leaves move, and then the deception will be to him a deception no longer, and he will gobble up the poor insect.

In our attempts to discover the planets we experience just the same difficulties as the insect-eating bird. Wide as is the true difference between a planet and a star, there is yet such a seeming resemblance between them that we are often puzzled to know which is which. The planets imitate the stars so successfully, that when one of them is presented to us among myriads of stars it is impossible for us to detect the planet by its appearance. But we can be cunning--we can steadily watch, and the moment we find one of these star-like points beginning to creep about we can pounce upon it. We know by its movements that it is only disguised as a star, but that it is really one of the planets.

It is not always easy to discover the asteroids even by this principle, for unfortunately these bodies move very slowly. If you have a planet in the field of view, it will creep along so gradually that an hour or more must have elapsed before it has shifted its position with respect to the neighboring stars to any appreciable extent. The search for such little planets is therefore a tedious one, but there are two methods of conducting it: the new one, which has only recently come into use, and the old one. I shall speak of the old one first.

Although the body’s motion is so slow, yet when sufficient time is allowed, the planet will not only move away from the stars close by, but will even journey round the entire heavens. The surest way of making the discovery is to study a small part of the heavens now and to examine the same locality again months or years afterwards. Memory will not suffice for this purpose. No one could recollect all the stars he saw with sufficient distinctness to be confident that the field of the telescope on the second occasion contained either more or fewer stars than it did on the first. The only way of doing this work is to draw a map of the stars very carefully. This is a tedious business, for the stars are so numerous that even in a small part of the heavens there will be many thousands of stars visible in the telescope. All of these will have to be entered faithfully in their true places on the map. When this has been done the map must be laid aside for a season, and then it is brought out again and compared with the sky. No doubt the great majority of the objects will be found just as they were before. These are the stars, the distant suns, and our concern is not at present with them. Sometimes it will happen that an object marked on the first map has left a vacant place on the second. This, however, does not help us much, for, whatever the object was, it has vanished into obscurity, and a new planet could hardly be discovered in this way. But sometimes it will happen that there is a small point of light seen in the second map which has no corresponding point in the first. Then, indeed, the expectation of the astronomer is aroused; he may be on the brink of a discovery. Of course he watches accurately the little stranger. It might be some star that had been accidentally overlooked when forming the map, or it might possibly be a star that has become bright in the interval. But here is a ready test: is the body moving? He looks at it very carefully, and notes its position with respect to the adjacent stars. In an hour or two his suspicions may be confirmed; if the object be in motion, then it is really a planet. A few further observations, made on subsequent days, will show the path of the body. And the astronomer has only to assure himself that the object is not one of the planets that have been already found before he announces his discovery.

The new method of searching for small planets, which has only come into use in recent years, is a very beautiful one, and renders the process of making such discoveries much more easy than the older method which I have just described.

We can take photographs of the heavenly bodies by adjusting a sensitive plate in the telescope so that the images of the objects we desire to see shall fall upon it. The method will apply to very small stars, if by excellent clockwork and careful guiding we can keep the telescope constantly pointing to the same spot until the stars have had time to imprint their little images. Thus we obtain a map of the heavens, made in a thoroughly accurate manner. Indeed, the delicacy of photography for this purpose is so great, that the plates show many stars which cannot be seen with even the greatest of telescopes. Suppose that a little planet happened to lie among the stars which are being photographed. All the time that the plate is being exposed the wanderer is, of course, creeping along, and after an hour (exposures even longer are often used), it may have moved through a distance sufficient to ensure its detection. The plate will, therefore, show the stars as points, but the planet will betray its presence by producing a streak.

The asteroids now known number between 400 and 500. Out of this host a few afford some information to the astronomer, but the majority of them are objects possessing individually only the slightest interest. No small planet is worth looking at as a telescopic picture. We should consider that asteroid to be a large one which possessed a surface altogether as great as England or France. Many of these planets have a superficial extent not so large as some of our great counties. A globe which was just big enough to be covered by Yorkshire--if you could imagine that large county neatly folded round it--would make a very respectable minor planet.

We know hardly anything of the nature of these small worlds, but it is certain that any living beings they could support must have a totally different nature from the creatures that we know on this earth. We can easily prove this by making a calculation. I shall suppose a small planet one hundred miles wide, its diameter being, therefore, the one-eightieth part of the diameter of the earth. If we were landed on such a globe, we should be far more puzzled by the extraordinary lightness of everything than we should be in the similar case of the moon to which I referred (p. 124). If we suppose the planet to be constructed of materials which had the same density as those of which the earth is made, then every weight would be reduced to the eightieth part of what it is here.

There would be one curious consequence of residence on such a globe. We have heard of attempts to make flying machines, or to provide a man with wings by which he shall soar aloft like the birds. All such contrivances have hitherto failed. It may be possible to make a pair of wings by which a man can fly down, but it is quite another matter when he tries to fly up again. Suppose, however, we were living on a small planet, it would be perfectly easy to fly, for as our bodies would only seem to weigh a couple of pounds, we ought to be able to flap a pair of wings strong enough to overcome so trivial a force. I should, however, add that this is on the supposition that the atmosphere has the same density as our own.

Life on these small planets would indeed be extraordinary. Let us take, for example, Flora, and see how a game of lawn tennis on that body would be managed. The very slightest blow of the racket would drive the ball a prodigious distance before it could touch the ground; indeed, unless the courts were about half a mile long, it would be impossible to serve any ball that was not a fault. Nor is there any great exertion necessary for playing lawn tennis on Flora, even though the courts are several hundred acres in extent. As a young lady ran to meet the ball and return it, each of her steps might cover a hundred yards or so without extra effort; and should she have the misfortune to get a fall, her descent to the ground would be as gentle as if she was seeking repose on a bed of the softest swan’s-down.

These little planets cluster together in a certain part of our system. Inside are the four inner planets, of which we have already spoken; outside are the four outer planets, of which we have soon to speak. Between these two groups there was a vacant space. It seemed unreasonable that where there was room for planets, planets should not be found. Accordingly the search was made, and these objects were discovered. Even at the present day, more and more are being constantly added to the list.

Up till quite recently all the small planets which had been discovered confined themselves to the space lying between the paths of the major planets Mars and Jupiter. This invariable rule was, however, departed from in the case of one of these bodies which was discovered in August, 1898. This little body, which was known for some time by the provisional appellation of D Q, and which has now been definitely christened Eros, is an exception to this rule. It travels at an average distance from the sun actually less than that of Mars, and at the nearest point can come within 15,000,000 miles of the earth.

We occasionally get information from these little bodies; for in their revolutions through the solar system, they sometimes pick up scraps of useful knowledge, which we can elicit from them by careful examination. For example, one of the most important problems in the whole of astronomy is to determine the sun’s distance. I have already mentioned one of the ways of doing this, which is given by the transit of Venus. Astronomers never like to rely on a single method; we are therefore glad to discover any other means of solving the same problem. This it is which the little planets will sometimes do for us. Juno on one occasion approached very close to the earth, and astronomers in various parts of the globe observed her at the same time. When they compared their observations they measured the sun’s distance. But I am not going to trouble you now with a matter so difficult. Suffice it to say, that for this, as for all similar investigations, the observers were constrained to use the very same principle as that which we illustrated in Fig. 5.

Let me rather close this lecture with the remark that we have here been considering only the lesser members of the great family which circulate round the sun, and that we shall speak in our next lecture of the giant members of our system.

LECTURE IV.

JUPITER, SATURN, URANUS, NEPTUNE.

Jupiter, Saturn, Uranus, Neptune--Jupiter--The Satellites of
Jupiter--Saturn--The Nature of the Rings--William Herschel--The
Discovery of Uranus--The Satellites of Uranus--The Discovery of
Neptune.

Our lecture to-day ought to make us take a very humble view of the size of our earth. Mercury, Venus, and Mars may be regarded as the earth’s peers, though we are slightly larger than Venus, and a good deal larger than Mercury or Mars; but all these four globes are insignificant in comparison with the gigantic planets which lie in the outer parts of our system. These great bodies do not enjoy the benefits of the sun to the same extent that we are permitted to do; they are so far off that the sun’s rays become greatly enfeebled before they can traverse the distance; but the gloom of their situation seems to matter but little, for it is highly improbable that any of these bodies could be inhabited.

A view of parts of the paths of these four great planets is shown in Fig. 65. The innermost is Jupiter, which completes a circuit in about twelve years; then comes Saturn, revolving in an orbit so great that twenty-nine years and a half are required before the complete journey is finished. Still further outside is Uranus, which has a longer journey than Saturn, and moves so much more slowly that a man would have to live to the ripe old age of eighty-four if a complete revolution of Uranus was to be accomplished during his lifetime. At the boundary of our system revolves the planet Neptune, and though it is a mighty globe, yet we cannot see it without a telescope. It is invisible to the naked eye for two reasons: first of all, because it is so far from the sun that the light which illuminates it is excessively feeble; and, secondly, because it is so far from us that whatever brilliancy it has is largely reduced.

JUPITER.

Of all these bodies Jupiter is by far the greatest; he is, indeed, greater than all the other planets rolled into one. The relative insignificance of the earth when compared with Jupiter is well illustrated by the fact that if we took 1200 globes each as big as our earth, and made them into a single globe, it would only be as large as the greatest of the planets. A view of the comparative sizes of the earth and Jupiter is shown in Fig. 66.

Fig. 67 shows a picture of Jupiter as seen through the telescope. First, you will notice that the outline of the planet’s shape is not circular, for it is plain that the vertical diameter in this picture is shorter than the horizontal one; in fact, Jupiter is flattened at the Poles and bulges out at the equator, so that a section through the Poles is an ellipse. Jupiter is turning round rapidly on his axis, and this will account for the protuberance. We find that the planet has assumed almost the same form as if it were actually a liquid. This we can illustrate by a globe of oil which is poised in a mixture of spirits of wine and water so carefully adjusted that the oil has no tendency to rise or fall. As we make the globe of oil rotate, which we can easily do by passing a spindle through it, we see that it bulges out in the form that Jupiter as well as other planets have taken.

On the picture of the planets you will see shaded bands. These are constantly changing their aspect, and for a double reason. In the first place, they change because Jupiter is rotating so quickly that in five hours the whole side of the planet which is towards us has been carried out of sight. In another five hours the original side of the globe will be back again, for the entire rotation occupies about 10 hours, or, more precisely, 9 hours 55 minutes 21 seconds.

But these bands are themselves not permanent objects. They have no more permanence than the clouds over our own sky. Sometimes Jupiter’s clouds are more strongly marked than on other occasions. Sometimes, indeed, they are hardly to be seen at all. It is from this we learn that those markings which we see when we look at the great planet are merely the masses of cloud which surround and obscure whatever may constitute his interior.

There is a circumstance which demonstrates that Jupiter must be an object exceedingly different from the earth, though both bodies agree in so far as having clouds are concerned. What would you think when I tell you that we were able to weigh Jupiter by the aid of his little moons, of which I shall afterwards speak? These little bodies inform us that Jupiter is about 300 times as heavy as our earth, and we have no doubt about this, for it has been confirmed in other ways. But we have found by actual measurement that Jupiter is 1200 times as big as the earth, and therefore, if he were constituted like the earth, he ought to be 1200 times as heavy. This is, I think, quite plain; for if two cakes were made of the same material, and one contained twice the bulk of the other, then it would certainly be twice as heavy. If there be two balls of iron, one twice the bulk of the other, then, of course, one has twice the weight of the other. But if a ball of lead have twice the bulk of a ball of iron, then the leaden ball would be more than twice as heavy as the iron, because lead is the heavier material. In the same way, the weights of the earth and Jupiter are not what we might expect from their relative sizes. If the two bodies were made of the same materials and in the same state, then Jupiter would be certainly four times as heavy as we find him to be. We are, therefore, led to the belief that Jupiter is not a solid body, at least in its outer portions. The masses of cloud which surround the planet seem to be immensely thick, and as clouds are, of course, light bodies in comparison with their bulk, they have the effect of largely increasing the apparent size of Jupiter, while adding very little to his weight. There is thus a great deal of mere inflation about this planet, by which he looks much bigger than his actual materials would warrant if he were constituted like the earth.

These facts suggest an interesting question. Why has Jupiter such an immense atmosphere, if we may so call it? The clouds we are so familiar with down here on the earth are produced by the heat of the sun, which beats down upon the wide surface of the ocean, evaporates the water, and raises the vapor up to where it forms the clouds. Heat, therefore, is necessary for the formation of cloud; and with clouds so dense and so massive as those on Jupiter, more heat would apparently be necessary than is required for the moderate clouds on this earth. Whence is Jupiter to get this heat? Have we not seen that the great planet is far more distant from the sun than we are? In fact, the intensity of the sun’s heat on Jupiter is not more than the twenty-fifth part of what we derive from the same source. We can hardly believe that the sun supplied the heat to make those big clouds on the great planet; so we must cast about for an additional source, which can only be inside the planet itself. So far as his internal heat is concerned, Jupiter seems to be in much the same condition now as our earth was once, ages ago, before its surface had cooled down to the present temperature. As Jupiter is so much larger than the earth, he has been slower in parting with his heat. The planet seems not yet to have had time to cool sufficiently to enable water to remain on his surface. Thus the internal heat of the planet supplies an explanation of his clouds. We may also remark that as the present condition of Jupiter illustrates the early condition of our earth, so the present condition of the earth foreshadows the future reserved for Jupiter when he shall have had time to cool down, and when the waters that now exist in the form of vapor shall be condensed into oceans on his surface.

THE SATELLITES OF JUPITER.

Every owner of a telescope delights to turn it on the planet Jupiter, both for the spectacle the globe itself affords him, and for a view of the wonderful system of moons by which the giant planet is attended. Fortunately the four satellites of Jupiter lie within reach of even the most modest telescope, and their incessant changes relative to Jupiter and each other give them a never-ending interest for the astronomer. Compared with the torpid performance of our moon, which requires a month to complete a circuit around the earth, Jupiter’s moons are wonderfully brisk and lively. Nor are they small bodies like the satellites of Mars, for the second of Jupiter’s satellites is quite as big as our moon, and the other three are very much larger. It is, however, true that his satellites appear insignificant when compared with Jupiter’s own enormous bulk.

The innermost of these little bodies flies right round in a period of one day and eighteen or nineteen hours, while the outermost of them takes a little more than a fortnight--that is, rather more than half the time that our moon demands for a complete revolution. Jupiter’s satellites are too far off for us to see much with respect to their structure or appearance even with mighty telescopes. It is, of course, their great distance from us that makes them look insignificant. They would, however, be bright enough to be seen like small stars were it not that, being so close to Jupiter, his overpowering brightness renders such faint objects in his vicinity invisible.

It was by means of the satellites of Jupiter that one of the most beautiful scientific discoveries was made. As a satellite revolves round the giant planet it often happens that the little body enters into the shadow of the great planet. No sunlight will then fall upon the satellite, and as it has no light of its own, it disappears from sight until it has passed through the shadow and again receives sunlight on the other side. We can watch these eclipses with our telescopes, and there can be no more interesting employment for a small telescope. The movements of these bodies are now known so thoroughly that the occurrence of the eclipses can be predicted. The almanacs will tell when the satellite is calculated to disappear, and when it ought again to return to visibility. When astronomers first began to make these computations a couple of hundred years ago, the little satellites gave a great deal of trouble. They would not keep their time. Sometimes they were a quarter of an hour too soon, and sometimes a quarter of an hour too late. At last, however, the reason for these irregularities was discovered, and a wonderful reason it was.